The present invention relates to FM signal demodulating circuits, and more particularly to an FM signal demodulating circuit of peak differential type which has a demodulating characteristic curve extending over a plurality of frequency bands and demodulates a plurality of FM signals.
As one type of circuit for demodulating a signal in which a carrier is frequency modulated by a modulating signal (such a signal being referred to as an FM signal in the present specification and appended claims), there have been heretofore known FM signal demodulating circuits of peak differential type. Further, there has been known an FM signal demodulating circuit which employs an FM signal demodulating circuit of peak differential type and has a circuit construction for selectively demodulating one of a plurality of FM signals having different carrier frequencies, as described hereinbelow.
In this conventional FM signal demodulating circuit, a circuit consisting of a first capacitor and a first switch connected in series and another circuit consisting of a second capacitor and a second switch connected in series are connected in parallel between two input terminals of a peak differential type amplifier. A circuit consisting of a third capacitor and a coil connected in parallel is further connected in parallel with these series connection circuits. A fourth capacitor is connected between one of the input terminals of the amplifier and the ground.
In this conventional FM signal demodulating circuit, parallel-connection of the first and second capacitors with the third capacitor and coil is made and broken by opening and closing of the first and second switches, whereby the value of the capacitance connected in parallel with the coil is changed.
Accordingly, when the first and second switches are opened, the demodulating circuit demodulates an FM signal having a carrier of a frequency corresponding to a high parallel resonance frequency determined by the capacitance of the third capacitor and the inductance of the coil. When the first and second switches are closed, the demodulating circuit demodulates an FM signal having a carrier of a frequency corresponding to a low parallel resonance frequency determined by the capacitances of the first, second and third capacitors and the inductance of the coil. When either one of the first and second switches is opened and the other one is closed, either one of the first and second capacitors is connected in parallel with the third capacitor and coil, whereby the demodulating circuit demodulates an FM signal having a carrier of a frequency corresponding to a parallel resonance frequency of the parallel connection circuit. As described above, the demodulating circuit selectively demodulates one FM signal out of four FM signals respectively having different carrier frequencies responsive to the combination in which the first and second switches are opened and/or closed.
In television systems, the frequency difference between the video carrier frequency and the audio sub-carrier frequency is 4.5 MHz in NTSC system, 5.5 MHz in CCIR system, 6 MHz in PAL-I system, and 6.5 MHz in SECAM-OIRT system. Frequencies of the color sub-carriers in the respective color television systems are approximately the same as are the band widths of video signals therein.
In this connection, when this conventional FM signal demodulating circuit is employed in a dual-system television receiver which is capable of selectively receiving television signals of different systems, it is sufficient to provide only one such demodulating circuit for the FM audio signals so that the circuit construction of the demodulating circuit system in the television receiver is simplified.
However, since this conventional FM signal demodulating circuit employs a plurality of switching elements, it has disadvantages in that troublesome manufacturing operations are required for mounting the switches on the cabinet of the television receiver and that the number of circuit elements is increased, whereby the manufacturing costs are increased. Further, it is troublesome for the viewer to switch the switching elements manually. Moreover, the slope or gradient of the demodulating characteristic curve of the S-figure is changed in response to making and breaking of the parallel connection of the first and second capacitors with the parallel connection circuit of the third capacitor and coil responsive to switching of the switches, whereby the demodulated output level is accordingly changed. There has been a further disadvantage that, since only the capacitance value is changed, linearity of the slope of the S-figure demodulating characteristic curve is poor giving rise to demodulation distortion.